1. Field of the Invention
The present invention relates generally to a hydraulic control device for a continuously variable transmission of a belt-and-pulley type, and more particularly to techniques for avoiding an unnecessary rise in fluid pressure in a hydraulic circuit for operating the transmission, thereby minimizing power loss.
2. Related Art Statement
A continuously variable transmission of a belt-and-pulley type, comprising an input and an output variable-diameter pulley provided on an input or drive shaft and an output or driven shaft, respectively; a transmission belt connecting the input and output variable-diameter pulleys; and two hydraulic cylinders to change effective diameters of the input and output pulleys is known. In such a continuously variable transmission, a ratio of a thrust of the output-side hydraulic cylinder to that of the input-side hydraulic cylinder must be changed over a relatively wide range, for example from 1.5 to 0.5, in order to change a speed ratio of the transmission to change output speeds over a wide range.
For such a continuously variable transmission, a hydraulic control system is known, wherein first and second line pressures are controlled by first and second pressure regulating valves, respectively. The first line pressure is lower than the second line pressure and is applied to the output-side hydraulic cylinder, primarily for controlling a tension of the transmission belt. The comparatively high second line pressure is applied to a flow control valve which controls the flow of hydraulic fluid to and from the input-side hydraulic cylinder, thereby changing the speed ratio of the transmission. In this type of hydraulic control system, a difference between the first and second line pressures assures a relatively large difference in thrust between the two cylinders, even if the two cylinders have substantially the same pressure receiving areas. An example of such hydraulic control system is disclosed in Japanese Patent Application which was published in 1973 under Publication No. 48-26692 for opposition purposes.